The present invention relates to a rolling bearing comprising an outer ring, an inner ring and a rolling element rotatably disposed therebetween. The rolling bearing may further comprises a cage. More particularly, the present invention relates to a rolling bearing which can work for a prolonged life time even if it is placed under an environment liable to contamination of lubricant by water content.
It is known that a rolling bearing normally exhibits a drastically deteriorated durability when the lubricant used is contaminated by water content. For example, it was reported that the fatigue life of a rolling bearing shows a drop of from a fraction to about one twentieth when the lubricant used is contaminated by water content in an amount of 6% as compared with when the lubricant is free of water content (Kyozaburo Yoshimura, Shinichi Shirota, Kiyoshi Hirakawa, "Rolling fatigue of surface starting point and inner starting point", NSK Bearing Journal, NO. 636, pp. 1-10, 1977 (hereinafter referred to as "Reference 1")).
It is obvious from Reference 1 that the contamination of lubricant by water content has a great effect on the life (durability) of rolling bearing. Thus, various techniques for preventing the contamination of lubricant by water content have heretofore been studied and developed according to the foregoing purpose of rolling bearing.
An example of rolling bearing which is used on the supposition that the lubricant can be contaminated by water content is a work rolling bearing for steel material rolling mill. In the prior art, the work rolling bearing is incorporated in a chock (bearing box) fitted with a contact rubber seal to prevent the entrance of a large amount of rolling water into the chock (bearing box) and hence the contamination of the lubricant sealed in the bearing by water content. However, when the contact rubber seal is deteriorated or damaged, water enters into the chock (bearing box), resulting in the contamination of the lubricant in the bearing by water content. In recent years, therefore, a technique in which a contact rubber seal is fit to the interior of a bearing as well as the exterior of the bearing to avoid the contamination of the lubricant by water content has been proposed (K. YAMAMOTO, M. YAMAZAKI, M. AKIYAMA, K. FURUMURA, "Introducing of Sealed Bearings for Work Roll Necks in Rolling Mills", Proceedings of the JSLE International Tribology Conference, pp. 609-614, Jul. 8-10, 1985, Tokyo, Japan (hereinafter referred to as "1st Prior Art")).
According to 1st Prior Art, the combined use of a contact rubber seal fitted in the chock (bearing box) outside the bearing and a contact rubber seal fitted in the interior of the bearing makes it possible to reduce the water concentration in the lubricant from 40% to less than 10% and the consumption of the lubricant to 1/200 as compared with the case where the entrance of water content is prevented only by a contact rubber seal fitted in the chock (bearing box). It is also reported that this arrangement resulted in the reduction of breakage of bearing to none as compared with the prior art, where several such accidents have been reported every year.
As another technique for preventing the contamination of the lubricant in the foregoing work roll bearing by water content there has been proposed a technique involving the supply of a lubricant into the chock (bearing box) with compressed air as a carrier gas (NSK Technical Journal No. 654, pp. 54-56, 1992 (hereinafter referred to as "2nd Prior Art")).
In accordance with 2nd Prior Art, the predetermination of the air pressure in the chock (bearing box) to a high value by the use of compressed air makes it possible to inhibit the contamination of the lubricant by water content.
The foregoing 1st Prior Art allows the reduction of the water concentration in the lubricant from 40% to less than 10% and the consumption of the lubricant as mentioned above. The results of use of work roll bearings have been studied. As a result, it was made obvious that the occurrence of seizing is drastically reduced but the working time which can last until peeling occurs, i.e., life L of the bearing is not improved too much. The reason for the reduction of occurrence of seizing is that the contact rubber seal incorporated in the bearing allows the reduction of the leakage of the lubricant to the exterior. The reason why the life F of the bearing is not improved too much is that the contamination of the lubricant by water content causes a drastic reduction of the resistance of the bearing to rolling fatigue.
It is reported that even when the lubricant is contaminated by water content in an amount as slight as about 100 ppm, the resistance of the bearing material to rolling fatigue shows a drop as large as 32 to 48% (P. Schatsberg, I. M. Felsen, "Effects of water and oxygen during rolling contact lubrication, wear", 12, pp. 331-342, 1968 (hereinafter referred to as "Reference 2"). If the contact rubber seal fitted in the chock (bearing box) outside the bearing and the contact rubber seal incorporated in the bearing are used in combination, the water concentration in the lubricant can be suppressed to less than 10% but the contamination of the lubricant by water content cannot be completely inhibited, making it impossible to avoid the reduction of the resistance of the bearing material to rolling fatigue as pointed out by Reference 2. In other words, 1st Prior Art is disadvantageous in that it cannot completely inhibit the contamination of the lubricant by water content, causing a drop of the resistance of the bearing material to rolling fatigue and hence making it impossible to provide the bearing with a desired durability, i.e., life L.
Further, in accordance with 2nd Prior Art, the contamination of the lubricant by water is inhibited by the predetermination of air pressure in the chock (bearing box) to a high value. Therefore, this effect doesn't depend on the waterproofness of the contact rubber seal as in 1st Prior Art. However, 2nd Prior Art is disadvantageous in that it can hardly perform almost perfect prevention of contamination by water content such as reduction of the water content in the lubricant to not more than 100 ppm.